Steer-by-wire steering systems are steering systems in which the positive mechanical coupling between the steering wheel and the steering gear that is compulsory with conventional vehicle steering systems during the operation is disabled. The steering movement introduced by the driver into the steering wheel is then detected by a sensor, processed by a control and converted by an actuating drive as a function of the control into a steering angle of a steered wheel. On the part of the driver, a manual torque is applied to the steering wheel by means of a manual torque adjuster in order to simulate the actuation forces and reaction forces that are required for a safe driving.
With such steering systems it is possible to perform for example automatic cross wind corrections, counter steering and other steering operations without the steering wheel having to follow these steering movements. Operations of the autonomous driving can also be performed without the steering wheel turning in the process.
The requirements in terms of failure safety of such a steering system are particularly high. Accordingly, the steerability of the vehicle has to be ensured in any possible case of malfunctioning. For this purpose, redundancies and fallback levels are provided, for example in the case of systems with electromechanical actuating drive and electromechanical manual torque adjuster. With these systems, the steering shaft is disconnected between the two drives by means of a mechanical coupling in order to make possible the steer-by-wire function during the operation. In the event of a malfunctioning, the coupling is closed in order to establish a positive mechanical coupling between steering wheel and steering gear, as a result of which the steering system then remains actuatable similar to a conventional steering system and the vehicle can at least be brought safely to a halt.
A generic electromechanical steer-by-wire steering system is known from the document WO2014/038452A1. Here, the manual torque adjuster is arranged on the steering column in the vicinity of the steering wheel. The steering column is coupled, via a steering shaft, to a steering gear on which the electromechanical actuating drive, as in the case of an electromechanical power steering, is also arranged. The steering shaft can be selectively disconnected or connected by means of a coupling that is arranged between an upper part and a lower part. The disconnected state with opened coupling in this case is the operating state while the connected state with closed coupling represents the mechanical fallback level.
The arrangement is complicated in the manufacture and assembly because the steering column with the manual torque adjuster, the steering gear with the actuating drive and the steering shaft with the coupling are separate assemblies which have to be manufactured and assembled and which, above all, have to be connected to one another via electrical lines during the operation in such a manner that a particularly high malfunctioning and failure safety is achieved. Such a steering system is deemed expensive for using such steering systems in light and economical vehicles such as for example small and compact vehicles.
Thus a need exists for a steer-by-wire steering system that is less expensive to manufacture and assemble.